A mysterious title for what is, essentially, a series of polite calls on the surviving eminences of the (principally American) 20th-century musical avant garde. High points include a view of a rehearsing Terry Riley, and a chat with his fellow epic beard, La Monte Young, who indulges in some throat singing. Prism’s guiding voice, seen preparing a piano with John Cage and heard opining on each of the artists discussed, is filmmaker Jacqueline Caux’s late husband, musicologist and proselytizer Daniel Caux, one of those delightfully encyclopedic Frenchmen whose particular subject was “other musics,” including North American underground work of the experimental, improvisational, drone, minimalist, and-so-on persuasions. Aside from some interim scenes dwelling on broody cityscapes, the presentation is straightforward, the interviews candid and familiar, with appeal for both novices and disciples. One wishes, though, for more vintage footage, as is seen in Caux’s also-playing portrait tribute to the dancer-choreographer Anna Halprin, who has collaborated with many of Prism’s musicians. Without this, the subject seems excessively institutional, antique. Attempting to vivify things is a final associative leap, going from a conversation with composer Gavin Bryars, talking of madrigals, straight into the Detroit techno scene, the subject of Caux’s The Cycles of the Mental Machine.